The purpose of this application is to use social epidemiological methods to prospectively evaluate the impact of social, environmental, and structural factors on partner-level sexual and drug risk patterns and HIV and sexually transmitted infection (STI) incidence among FSWs and to assess the feasibility of a subsequent HIV/STI intervention targeting FSWs in public place-based settings (e.g. bars, streets). Despite growing evidence of the effectiveness of environmental-structural HIV/STI interventions targeting FSWs, this research has been largely derived from developing country settings and establishment-based venues, and there is limited prospective data on the social and structural context of HIV/STI risk among FSWs in North America. Given recent reports suggest a temporal shift from an injection to a sexually-driven HIV epidemic among street-involved populations in many North American settings, there is a crucial need for more prospective data on the contextual factors promoting and constraining partner-level sexual and drug risk patterns and HIV/STI acquisition. Vancouver, Canada is ideally suited for the proposed study for several reasons. First, due to the quasi-criminalized nature of sex work and large informal tolerance zones, we are in a unique position to evaluate the context of HIV/STI risk among FSWs, not currently feasible in most US cities. Second, unlike many urban environments where the FSW community is fragmented, the current social and legal context of sex work in Canada, our ability to harness strong community FSW support for this application, and new supportive housing models currently operating legally as quasi-brothels demonstrate significant innovation and strong potential for a subsequent intervention stemming from the results of this research. Herein we propose to create an open prospective cohort of 500 active FSWs working in public place-based settings, including interview administered questionnaires and HIV/STI screening at baseline and 6monthly follow-up over a five year period. To our knowledge this study is among the first prospective studies of public place-based sex work in North America and as such, we will be uniquely positioned to evaluate the risk environment and temporal trends in partner-level risks and HIV/STI acquisition among FSWs. This research will address several questions central to the urgent challenges facing HIV/STI prevention capacity in North America and will directly assess the feasibility of a subsequent intervention that will be of critical importance to other developed country settings.